(re)proposing a mainEntity property for schema.org

I've taken another run at writing up the mainEntity problem. The goal is to
add a simple property definition that can improve the ability to determine
which of several described entities is the main one being described in some
document.

https://github.com/schemaorg/schemaorg/issues/301

Any thread that touches on urls, standards for identifiers and real world
entities carries a strong risk of dissapearing into a giant
https://en.wikipedia.org/wiki/HTTPRange-14 mess. I'm hoping we can find a
shortcut towards doing something useful even without solving all related
problems...

Thanks for any comments. I'd suggest commenting in Github over a mail
thread, but either is fine of course.

cheers,

Dan